1. Field of the Invention
The present invention relates to a control unit for controlling a DC/DC converter and a DC/DC converter including the control circuit. More particularly, the present invention is concerned with a start control method for a non-isolated DC/DC converter that is applied to a power feed circuit that feeds power to a load from a non-isolated DC/DC converter, which is realized with a compact non-isolated on-board power supply or the like, disposed near the load so as to prevent a voltage drop caused by a wiring resistance.
2. Description of the Related Art
In recent years, a power feed circuit that feeds power by a compact non-isolated on-board power supply disposed near a load so as to cope with a voltage drop caused by a wiring resistance, included in order to lower an operating voltage, has generally prevailed. FIG. 1 shows an example of a power feed circuit including a non-isolated on-board power supply.
As shown in FIG. 1, an isolated power supply 1 transforms a primary voltage such as a mains voltage while insulating a primary side from a secondary side, and then supplies dc power to each of non-isolated on-board power supplies 10a to 10c. The non-isolated on-board power supplies 10a to 10c convert the dc power fed from the isolated power supply 1 into dc power of a desired voltage, and feed the dc power to loads 2a to 2c connected to the respective on-board power supplies. Hereinafter, the non-isolated on-board power supplies 10a to 10c may generically be called a non-isolated on-board power supply 10. Likewise, the loads 2a to 2c may generically be called a load 2.
A DC/DC converter exemplified by the non-isolated on-board power supply in this specification is a power conversion circuit. In general, as long as an output power remains constant, an input voltage and an input current are inversely proportional to each other. FIG. 2 shows the relationship between the input voltage and input current. In FIG. 2, supposing that the non-isolated on-board power supply 10 feeds constant power, an input current Ii flowing from the isolated power supply 1 is inversely proportional to an input voltage Vi of the non-isolated on-board power supply 10. Therefore, if the output voltage of the isolated power supply 1 to be applied to the non-isolated on-board power supply 10 is a voltage V1 lower than a rated output voltage Vsr, a current I1 larger than an intended input current I2 flows into the non-isolated on-board power supply 10.
The foregoing property of the non-isolated on-board power supply 10 poses the problem described below. Namely, if a voltage rise, occurring when the isolated power supply 1 in a preceding stage initiates a power feed is moderate by reason of a large electrostatic capacity of a load imposed on the isolated power supply 1, before the voltage reaches the rated output voltage Vsr, the non-isolated on-board power supply 10 starts to receive a large current. As a result, a protective fuse may be melted or the isolated power supply 1 in the preceding stage may halt due to an overload. Referring to FIG. 3, this mechanism will be described below.
The first to fourth timing charts included in FIG. 3 indicate time-varying changes in an input voltage Vi, an input current Ii, an output voltage Vo, and an output current Io of the non-isolated on-board power supply 10. As seen from the first timing chart of the FIG. 3, the isolated power supply 1 initiates the power feed to the non-isolated on-board power supply 10 at a time instant t0. The output voltage of the isolated power supply 1 gradually rises until it reaches the rated output voltage Vsr at a time instant t2.
When the output voltage Vs of the isolated power supply 1 gradually rises, the non-isolated on-board power supply 10 starts with a starting voltage Via lower than the rated output voltage Vsr (time instant t1). As seen from the second timing chart of FIG. 3, a current I1 much larger than an input current I2 that flows with application of the rated output voltage Vsr flows into the non-isolated on-board power supply 10.
In efforts to solve the foregoing problem, a circuit for monitoring an input voltage as shown in FIG. 4 is conventionally included for restricting the input voltage Vi that causes an non-isolated on-board power supply to start. Specifically, voltage divider resistors R1 and R2 are used to produce a fraction of the input voltage Vi, and the fractional voltage is compared with a constant voltage Vc serving as a reference in order to turn on or off a switching element drive circuit 11. If a threshold for the input voltage Vi with which the switching element drive circuit 11 is turned on or off is set to a value near a rated voltage, production of a large current occurring at the start of the power supply 1 in the preceding stage can be prevented.